Moving-coil meters

Need to hunt down a problem lurking in an
electric circuit?
You'll need a meter of some kind, maybe even an oscilloscope. Most people use digital meters
these days that show readings of current, voltage, and resistance on an LCD display
(they're sometimes called solid-state or electronic meters). But many of us
still prefer the old kind of meter with a pointer that sweeps back
and forth on a dial. Moving-coil meters, as these things are known, are still
widely used in all kinds of different equipment, from airplane
cockpit instruments to sound-level (VU) meters in recording studios. Let's take
a closer look at how they work!

Photo: A typical high-current ammeter on a car battery charger. This one can indicate the approximate size of a current up to 6 amps (A), though the scale isn't marked precisely enough for accurate measurements.

Electricity makes magnetism

Moving-coil meters work in a similar way to electric motors.
If you know how one of those works, understanding a meter is easy. Either way, let's start from the
beginning. If you send an electric current down a metal wire, you briefly create a
magnetic field around the wire at the same time. You can't
see it but it's there nevertheless—and you can make it do some very
interesting things. Put a compass near a wire, switch on the current,
and you'll see the needle flick off course as you do so. Switch off the current
and the needle will flick again. Roughly speaking, this is the science at work in a moving coil meter: the
electric current passing down a wire creates a magnetic field that
makes a needle flick to one side. But how does that happen, exactly?

Inside a meter, a tight coil of copper wire, wrapped
round an iron core, is mounted in between the poles of a permanent
magnet. The coil has connections at either end so you can pass an
electric current through it and it has a long pointer stuck to it
that runs out across the meter dial. When you connect the meter into
a circuit and turn on the current, the current creates a magnetic
field in the coil. The field repels the magnetic field created by the
permanent magnet, making the coil rotate and turning the pointer up
the dial. The more current that flows through the coil, the bigger
the magnetic field it creates, the greater the repulsion, the more the
coil turns, and the further up the dial the pointer goes. So the
pointer gives you a measurement of how much current is passing through the coil.
With appropriate calibration, you can use the dial to measure the current directly.

Photo: James Clerk Maxwell summed up the relationship between
electricity and magnetism in his theory of electromagnetism, published in the mid-19th century.
All electromagnetic gadgets—from motors and generators to moving coil meters—are
based on science figured out by Maxwell and other early pioneers of electromagnetism.
Public domain photo by courtesy of Wikimedia Commons.

How moving-coil meters work

With the probes unconnected, the meter is like a circuit broken by an open switch: no current can flow into the meter or the coil inside it.

With no current flowing, the coil generates no magnetic field and the pointer stays at zero.

Connect the meter probes to something you're testing (such as a circuit board), and current will immediately start to flow through the meter and the coil inside it.

The moving current creates a temporary magnetic field around the coil that repels the magnetic field created by the permanent magnet. The strength of the magnetic field is directly related to the size of the current that flows through the coil.

The greater the current, the greater the magnetic field produced by the coil, and the higher up the dial the pointer moves.

It's worth noting briefly that the pointer acts like a lever, magnifying the movement at the
coil and producing a bigger deflection on the dial. In other words, if the coil moves by only a tiny amount, the pointer will move up the dial by a much greater amount that's easier to measure. That helps us make a more accurate measurement.

Different types of meters

Photo: This digital meter gives an instant reading of voltage, resistance, or current on an LCD display. You simply turn the dial in the center to convert it from one kind of meter to another. Currently it's set as a voltmeter and showing 1.31 volts DC (direct current). That's because I'm testing the voltage of the small battery you can see on the table just above the meter.

You can use moving-coil meters to measure voltage, current, or resistance—but
you have to connect them up in different ways in each case.

Voltmeters

To measure voltage, you connect a meter in parallel across the
two points of the circuit you want to measure. Voltage-measuring meters are called, not surprisingly,
voltmeters.

Ammeters

To measure current, you place your meter in series (insert it directly into the path of the
circuit). Current-measuring meters are generally called ammeters
(since they measure in amps)
or galvanometers (after Luigi Galvani,
the Italian who famously discovered electric current by making frogs' legs twitch). If large currents are being measured,
ammeters typically need an extra resistance called a shunt
fitted in parallel with their terminals. Most of the current flows
through the shunt, leaving only a small fraction flowing through the
meter coil itself (thus protecting the mechanism). Some ammeters have
dials on their box so you can measure a wide range of different
currents. Turning the dial effectively switches a different-sized
resistance into the measuring circuit, with bigger shunts used
to measure larger currents.

Ohmmeters

You can measure the resistance of a circuit in three ways. You can use an ammeter and
voltmeter to measure the current and voltage and then use Ohm's law. Or you can measure the
resistance in a single operation using a slightly different design
of moving-coil meter called an ohmmeter, which is effectively an
ammeter with its own built-in battery.
The battery provides a voltage of known size. When you place the meter probes across the resistance
you want to measure, you complete a circuit and a current flows. The
meter measures the size of this current, but
shows it as a resistance (the dial is calibrated in ohms based on the
fixed voltage of the battery inside the meter). You can make more
accurate measurements of resistance using a slightly more complex
type of circuit called a Wheatstone bridge.

Photo: Galvanometers have quite a lot in common with compasses, which also rely on a magnetic needle moving in a magnetic field. In this early galvanometer design from the 1880s, patented by Isaac Chisholm in 1888, the similarity is obvious: instead of a modern-style pointer and scale, we have a compass needle that swings around when you feed a current into the two wires at the front. Underneath the needle, in the large blue circular box, there's an electromagnet to which the wires are connected. You can find out more about this meter in US Patent 390,067: Galvanometer. Artwork courtesy of US Patent and Trademark Office.